NSM in Modal and Random Vibration

PSiedlaczek
PSiedlaczek Altair Community Member
edited August 2022 in Community Q&A

How to apply NSM to Modal analysis in Optistup ?
My current tries without success:
- In Load Steps definition no NSM field exist for modal
- Modal with Pretension load step does not transfer NSM mass from Pretension LC
- Iv try to use GLOBAL_CASE_CONTROL > NSM >
- Iv also tried to use GLOBAL_CASE_CONTROL > NSM > NSMADD >

I cannot find information that NSM cannot be used in Linear Dynamics so I suppose I do something wrong.? Any clue ?

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Answers

  • Robinson Ferrari_20451
    Robinson Ferrari_20451
    Altair Employee
    edited August 2022

    Hi PSiedlaczek,

    Apply NSM in GLOBAL CASE CONTROL. It should work for you.
    Please take a look at the files attached for reference.
    You can also apply directly to the property (for instance PSHELL).

    With NSM
    Volume = 8.67618E+01 Mass = 8.67625E-02
    Generalized Generalized
    Subcase Mode Frequency Eigenvalue Stiffness Mass
    1 1 1.224102E+01 5.915547E+03 5.915547E+03 1.000000E+00
    1 2 5.012952E+01 9.920802E+04 9.920802E+04 1.000000E+00
    1 3 6.650601E+01 1.746150E+05 1.746150E+05 1.000000E+00

    Without NSM
    Volume = 8.67618E+01 Mass = 1.11552E-01
    Generalized Generalized
    Subcase Mode Frequency Eigenvalue Stiffness Mass
    1 1 9.248436E+00 3.376730E+03 3.376730E+03 1.000000E+00
    1 2 3.973661E+01 6.233636E+04 6.233636E+04 1.000000E+00
    1 3 5.086994E+01 1.021603E+05 1.021603E+05 1.000000E+00

    Let me know if works for you, and then please don't forget to accept the answer as correct for helping others.

    Thank you very much,

    Robinson

  • PSiedlaczek
    PSiedlaczek Altair Community Member
    edited August 2022

    I see that your modal works with NSM when applied by GLOBAL_CASE_CONTROL. It will also apply NSM Mass to all Load Cases, but I have many Load scenarios and thus I want to apply NSM only to chosen LC

    There is also more differences:
    - You applied NSM to Property, when I use subset of Elements (so in Your case all elements are modified)
    - I use NSMADD to check influence in response when mass change location
    - I also solve static load cases where i use NSMADD or single NSM

    In such load scenario GLOBAL_CASE_CONTROL is not applicable.
    Please see the attached FEM which compare Point Mass and NSMADD

    So maybe NSM shall be typed manually in solver deck for Modal analysis ID=2 only ?

  • Robinson Ferrari_20451
    Robinson Ferrari_20451
    Altair Employee
    edited August 2022
    PSiedlaczek said:

    I see that your modal works with NSM when applied by GLOBAL_CASE_CONTROL. It will also apply NSM Mass to all Load Cases, but I have many Load scenarios and thus I want to apply NSM only to chosen LC

    There is also more differences:
    - You applied NSM to Property, when I use subset of Elements (so in Your case all elements are modified)
    - I use NSMADD to check influence in response when mass change location
    - I also solve static load cases where i use NSMADD or single NSM

    In such load scenario GLOBAL_CASE_CONTROL is not applicable.
    Please see the attached FEM which compare Point Mass and NSMADD

    So maybe NSM shall be typed manually in solver deck for Modal analysis ID=2 only ?

    Hi PSiedlaczek

     

    You are right, you can specify in the solver deck for each load step separately (as you have done in your deck file). Some features can be available only in the deck files sometimes (OptiStruct is updated faster than HM).

    $HMNAME LOADSTEP 1"LC1+NSM" 1
    $
    SUBCASE 1
    LABEL LC1+NSM
    ANALYSIS STATICS
    SPC = 10
    LOAD = 2
    NSM = 1
    DISPLACEMENT = ALL
    $


    $HMNAME LOADSTEP 2"modal1+NSM ??" 3
    $
    SUBCASE 2
    LABEL modal1+NSM ??
    ANALYSIS MODES
    SPC = 10
    METHOD(STRUCTURE) = 3
    NSM = 2 (here you should add NSM=2 using NSMADD to keep NSM=1).


    $
    $HMNAME LOADSTEP 3"LC2 w/o NSM" 1
    $
    SUBCASE 3
    LABEL LC2 w/o NSM
    ANALYSIS STATICS
    SPC = 10
    LOAD = 2
    DISPLACEMENT = ALL

  • Robinson Ferrari_20451
    Robinson Ferrari_20451
    Altair Employee
    edited August 2022
    PSiedlaczek said:

    I see that your modal works with NSM when applied by GLOBAL_CASE_CONTROL. It will also apply NSM Mass to all Load Cases, but I have many Load scenarios and thus I want to apply NSM only to chosen LC

    There is also more differences:
    - You applied NSM to Property, when I use subset of Elements (so in Your case all elements are modified)
    - I use NSMADD to check influence in response when mass change location
    - I also solve static load cases where i use NSMADD or single NSM

    In such load scenario GLOBAL_CASE_CONTROL is not applicable.
    Please see the attached FEM which compare Point Mass and NSMADD

    So maybe NSM shall be typed manually in solver deck for Modal analysis ID=2 only ?

    Hi PSiedlaczek,
    Please let me know if it works for you to type NSM manually in the solver deck. And, then mark the question as resolved to let us know we find the solution and help others.

    Thank you very much, 
  • PSiedlaczek
    PSiedlaczek Altair Community Member
    edited August 2022

    Iv edited manually the solver deck and ran it. Following error was returned.
    It looks that NSM card is not allowed in modal analysis definition in r2021.0 and can be applied to modal only by GLOBAL_CASE_CONTROL unfortunately.


     *** PROGRAM STOPPED: FATAL ERROR(s) ENCOUNTERED.

                            ***** END OF REPORT *****

     For Useful OptiStruct Tips and Tricks, go to the URL:
     http://www.altairhyperworks.com/tips.aspx

    ************************************************************************

     A fatal error has been detected during input processing:

      *** ERROR # 2359 ***
      NSM control card in Subcase 2 not allowed.
      NSM=n control cards allowed only in static subcases.


  • Robinson Ferrari_20451
    Robinson Ferrari_20451
    Altair Employee
    edited August 2022
    PSiedlaczek said:

    Iv edited manually the solver deck and ran it. Following error was returned.
    It looks that NSM card is not allowed in modal analysis definition in r2021.0 and can be applied to modal only by GLOBAL_CASE_CONTROL unfortunately.


     *** PROGRAM STOPPED: FATAL ERROR(s) ENCOUNTERED.

                            ***** END OF REPORT *****

     For Useful OptiStruct Tips and Tricks, go to the URL:
     http://www.altairhyperworks.com/tips.aspx

    ************************************************************************

     A fatal error has been detected during input processing:

      *** ERROR # 2359 ***
      NSM control card in Subcase 2 not allowed.
      NSM=n control cards allowed only in static subcases.


    Hi PSiedlaczek,

    Yes, all other solution sequences (but linear/nonlinear static analysis) should be specified globally.

    https://2022.help.altair.com/2022/hwsolvers/os/topics/solvers/os/nsm_sub_r.htm?zoom_highlight=nsm

    1. The NSM Subcase Entry is currently subcase-dependent only for Linear Static Analysis and Nonlinear Static Analysis. For all other solution sequences, the NSM entry should be defined globally above the first SUBCASE statement. If the NSM Subcase Entry is specified within any subcase which is not linear static or nonlinear static, then the run will be terminated with an error. Refer to Example 1 and Example 2 for a better understanding of current support for non-structural mass.

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